Friday, September 07, 2007

Arthur Eddington was innocent![This is, pre-edited as usual, my latest article for muse@nature. I wonder whether I have been a little guilty of the sin described herein, of over-enthusiastic demolition of the classic stories of science. In my 2005 book Elegant Solutions I made merry use of Gerald Geison’s sceptical analysis of the Pasteur discovery of molecular chirality; but Geison’s criticisms of the popular tale have themselves been controversial. All the same, his argument seemed to make sense to me, and I’m quite sure that there was indeed some myth-spinning around this tale, abetted by Pasteur himself to boost his own legend.]

Dismissing the famous ‘verification’ of Einstein’s general relativity as a work of data-fudging is unwarranted, a new study argues.

There was once a time when the history of science was conventionally told as a succession of Eureka moments in which some stroke of experimental or theoretical genius led the scales to fall from our eyes, banishing old, false ideas to the dustbin.

Now we have been encouraged to think that things don’t really happen that way, and that in contrast scientific knowledge advances messily, one theory vanquishing another in a process that involves leaps of faith, over-extrapolated results and judicious advertising. Antoine Lavoisioer’s oxygen theory, Friedrich Wöhler’s synthesis of urea and the ‘death of vitalism’, Louis Pasteur’s germ theory – all have been picked apart and reinterpreted this way.

Generally speaking, the picture that emerges is probably a more accurate reflection of how science works in practice, and is certainly preferable to the Whiggishness of classic popular ‘histories’ like Bernard Jaffe’s Crucibles: The Story of Chemistry. At its most extreme, however, this sceptical approach can lead to claims that scientific ‘understanding’ changes not because of any deepening insight into the nature of the universe but because of social and cultural factors.

One of the more recent victims of this revisionism is the ‘confirmation’ of Einstein’s theory of general relativity offered in 1919 by the British astronomer Arthur Eddington, who reported the predicted bending of light in observations made during a total ecplise. Eddington, it has been said, cooked his books to make sure that Einstein was vindicated over Newton, because he had already decided that this must be so.

This idea has become so widespread that even physicists who celebrate Einstein’s theory commonly charge Eddington with over-interpreted his data. In his Brief History of Time, Stephen Hawking says of the result that “Their measurement had been sheer luck, or a case of knowing the result they wanted to get.” Hawking reports the widespread view that the errors in the data were as big as the effect they were meant to probe. Some go further,saying that Eddington consciously excluded data that didn’t agree with Einstein’s prediction.

Is that true? According to a study by Daniel Kennefick, a physicist at the University of Arkansas [1], Eddington was in fact completely justified in asserting that his measurements matched the prediction of general relativity. Kennefick thinks that anyone now presented with the same data would have to share Eddington’s conclusion.

The story is no mere wrinkle in the history of science. Einstein’s theory rearranged everything we thought we knew about time and space, deepening his 1905 theory of special relativity so as to give a wholly new picture of what gravity is. In this sense, it transformed fundamental physics forever.

Crudely put, whereas special relativity dealt with objects moving at constant velocity, general relativity turned the spotlight on accelerating bodies. Special relativity argued that time and space are distorted once objects travel at close to the speed of light. This obliterated the Newtonian notion of an absolute reference frame with respect to which all positions, motions and times can be measured; one could only define these things in relative terms.

That was revolutionary enough. But in general relativity, Einstein asserted that gravity is the result of a distortion of spacetime by massive objects. The classic image, disliked by some physicists, is that of a cannonball (representing a star, say) on a trampoline (representing space time), creating a funnel-shaped depression that can trap a smaller rolling ball so that it circles like a planet in orbit.

Even light cannot ignore this remoulding of space by a massive body – the theory predicted that light rays from distant stars should be bent slightly as they skim past the Sun. We can’t hope to see this apparent ‘shifting’ of star positions close to the edge of the blazing Sun. But when it gets blotted out during a total solar eclipse, the bending should be visible.

This is what Eddington set out to investigate. He drew on two sets of observations made from equatorial locations during the eclipse of 29 May 1919: one at the town of Sobral in Brazil, the other on the island of Principe off Africa’s west coast.

With the technology then available, measuring the bending of starlight was very challenging. And contrary to popular belief, Newtonian physics did not predict that light would remain undeflected – Einstein himself pointed out in 1911 that Newtonian gravity should cause some deviation too. So the matter was not that of an all-or-nothing shift in stars’ positions, but hinged on the exact numbers.

The results from the two locations were conflicting. It has been claimed that those at Sobral showed little bending, and thus supported Newton, whereas those at Principe were closer to Einstein’s predictions. The case for prosecuting Eddington is that he is said to have rejected the former and concentrated on the latter.

This claim was made particularly strongly in a 1980 paper [2] by philosophers of science John Earman and Clark Glymour, whose position was made more widely known by Harry Collins and Trevor Pinch in their 1993 book The Golem [3]. Why would Eddington have done this? One possibility is that he had simply been won over by Einstein’s theory, and wanted to see it ‘proved’. But it’s also suggested that Eddington’s Quaker belief in pacifism predisposed him to see a British proof of a German theory as an opportunity for postwar reconciliation.

Kennefick has examined these claims in detail. It is true that the Principe data, which Eddington helped to collect himself, were poor: because of cloudy weather, there were only two useable photographic plates of star positions, with just five stars of each. When Eddington spoke about these measurements in a public talk in September, before he had had a chance to analyse them fully, he admitted that the deflection of starlight seemed to fall between the predictions of Newtonian and relativistic theories. He clearly needed the Sobral data to resolve the matter.

The latter came from two sets of astronomical measurements: one made with a so-called ‘Astrographic’ lens with a wide field of view, and the other using a 4-inch lens borrowed from the Royal Irish Academy. The Astrographic data were expected to be more reliable – and it seems that they supported the non-relativistic prediction. This is where the charges of data-fudging come in, because it has been asserted that Eddington ditched those results and focused instead on the ones collected with the 4-inch lens, which showed ‘full deflection’ in support of Einstein’s view.

The Sobral Astrographic data were discarded, for technical reasons which Dyson and Eddington described in their full account of the expeditions [4]. Kennefick argues that these reasons were sound – but he shows that in any case Eddington semed to have played no part in the decision. He was merely informed of the analysis of the Sobral plates by the expedition leader, the Astronomer Royal Frank Watson Dyson of the Greenwich Observatory in London. Dyson, however, was cautious of Einstein’s theory (as were many astronomers, who struggled to understand it), suspecting it was too good to be true. So it’s not obvious why he would fiddle with the data.

In any event, a modern analysis of these plates in 1979 shows that, taken together, they do support Einstein’s prediction rather well, and that the original teams made assumptions in their calculations that were justified even if they couldn’t be conclusively supported at the time.

Kennefick says that the ‘Eddington fudge’ story has mutated from the sober and nuanced analysis of Earman and Glymour to a popular view that the ‘victory’ of general relativity was nothing but a public-relations triumph. It is now sometimes cited as a reason why scientists should be distrusted in general. Kennefick admits that Eddington may well have had the biases attributed to him – but there is no evidence that he had the opportunity to indulge them, even if he had been so inclined.

It’s a salutary tale for all involved. Scientists need to be particularly careful that, in their eagerness to celebrate past achievements and to create coherent narratives for their disciplines, they do not construct triumphalist myths that invite demolition. (Crick and Watson’s discovery of the structure of DNA is shaping up as another candidate.)

But there is an undeniable attraction in exposing shams and parading a show of canny scepticism. In The Golem, Collins and Pinch imply that the ‘biases’ shown by Eddington are the norm in science. It would be foolish to claim that this kind of thing never happens, but the 1919 eclipse expeditions offer scant support for a belief that such preconceptions (or worse) are the key determinant of scientific ‘truth’.

The motto of the Royal Society – Nullius in verba, loosely translated as ‘take no one’s word for it’ – is often praised as an expression of science’s guiding principle of empiricism. But it should also be applied to tellings and retellings of history: we shouldn’t embrace cynicism just because it’s become cool to knock historical figures off their pedestals.

10 comments:

I'm glad you're interested in history. However, you seem unaware of recent work in the history of mathematics which is important in understanding Einstein as an advocate of natural mathematics. Above all, I recommend you read Garciadiego on Russell. Here is a comment on some of this work:

Dear, as a layman I have the following question on "Confirmation of Gravitational Bending of Light during 1919 Total Solar Eclipse (and subsequently) ". According to me, since bending is due to solar mass, it would always be there, at all time, eclipse or not. And since the position of the Sun is not changing (gravitationally) wrt to stars in 6 months (or long long time), why it was expected to change with and without Eclipse?

It is only the Earth (negligible role gravity wise) that is changing position! Changing Location of observer on Earth is not central to bending.